This invention relates to an improvement applicable to an avalanche photodiode (Hereinafter referred to as an APD), specifically, a multilayered ADP having a sensitivity in the wavelength range from 1.2 through 1.65 micrometers and having a light absorbing layer made of a III-V group compound or alloy semiconductor, such as, In.sub.x Ga.sub.1-x As.sub.1-y P.sub.y, GaSb et al. and a multiplication layer made of a III-V group compound or alloy semiconductor, such as, InP, Ga.sub.1-x Al.sub.x Sb et al. More specifically, this invention relates to an improved guard ring configuration thereof having a more reliable grade of performance and allowing the APD to be of the planar type.
An ADP is a type of photodiode which has various characteristics of improved performance, such as, higher grades of sensitivity and response time and lower values of dark-current and noise. These improved performance characteristics are realized by the avalanche breakdown phenomenon which occurs at a p-n junction arranged in a multiplication layer and applied with a reverse bias to cause the avalanche multiplication of the current of electrons and/or holes generated in a light absorbing region exposed to the light which has arrived at the light absorbing region. In order to realize these improved performance characteristics, it is required to allow breakdown to occur exclusively and uniformly in the central region of the diode where the light is absorbed. Therefore, APD's require some means which is effective to limit breakdown to the central region of the p-n junction.
The wavelength of light to which an APD is sensitive is determined by the amount of band gap Eg of the material with which the light absorbing layer thereof is made. In order to produce an APD sensitive to a longer wavelength of light, a layer combination of semiconductor, one of which has a relatively small amount of band gap Eg (to make itself a light abosorbing layer) and the other of which has a relatively large amount of band gap Eg (to make itself a multiplication layer), is used. For production of an APD sensitive to the light wavelength range of 1.2 through 1.65 micrometers, the combination of In.sub.x Ga.sub.1-x As.sub.1-y P.sub.y and InP and the combination of GaSb and Ga.sub.x Al.sub.1-x Sb are preferably employed.
Therefore, the APD's which are multilayered, employing the above referred III-V group compound or alloy semiconductors, and which are publicly known in the prior art, have the so-called self guard ring layer configuration, in which a p-n junction is formed. This p-n junction is formed for the purpose of the avalanche multiplication slightly above the heterojunction between the light absorbing layer of InGaAs, InGaAsP or the like and the multiplication layer of InP or the like, so that electric field intensity in the light absorbing layer becomes larger in the central region of the diode or the light-absorbed region than in the other region or the region which is not exposed to light. This causes the breakdown voltage of the p-n junction to become less in the central region of the diode which is exposed to light than in the region which is not exposed to light, thereby realizing the guard ring effect. The function of the so-called self guard ring configuration is based on the characteristic that the band gap Eg, of the III-V group compound or alloy semiconductor with which the light absorbing layer is made, is less than the band gap Eg of the III-V group compound or alloy semiconductor with which the active layer is made and that the breakdown voltage is less for the former than for the latter. It is publicly acknowledged that this so-called self guard ring configuration provides an excellent guard ring effect.
The inventors of the invention have, however, discovered that the APD's having the so-called self guard ring configuration involve a drawback described below. Since the electric field intensity is necessarily high in the light absorbing layer the tunnel current or the current component caused by the carriers directly excited by the high magnitude of electric field intensity inevitably occurs in this region, thereby causing various adverse effects e.g. large amounts of dark current, noise or the like. It is well-known that the magnitude of the tunnel effect is larger for a substance having a low band gap Eg, such as, the III-V group compound or alloy semiconductor e.g. InGaAsP, GaSb and the like. Incidentally, it is well-known that the fundamental absorption edge .lambda. is determined by an equality .lambda.=hc/Eg. In view of the fact that the signal transmission loss in an optical fiber is less for longer wavelength light and that APD's sensitive to longer wavelength light or APD's having a light absorbing layer made of a material having a low band gap Eg are increasingly preferable, the above-described drawback due to the large amount of tunnel current, which is inherent to APD's having the so-called self guard ring layer configuration, is significant.